Industrial process control systems are used to monitor and control industrial processes which produce or transfer liquids or the like. In such systems, it is typically important to measure “process variable” such as temperatures, pressures, flow rates, and others. Process control transmitters are used to measure such process variables and transmit information related to the measured process variable back to a central location such as a central control room.
One type of process variable transmitter is a pressure transmitter which measures pressures, or pressure differences between two pressures, of process fluids and provides an output related to the pressures or pressure differences. In the case of differential pressures, the pressure difference may, in turn, represent a flow rate, a level of a process fluid in a vessel, or other process variable. The pressure transmitter is configured to transmit the pressure information back to the central control room, typically via a two-wire process control loop. However, other techniques, such as wireless communication techniques may be used as well.
Process pressure transmitters generally sense pressure using a pressure sensor fluidically coupled to at least one isolation diaphragm. The isolation diaphragm isolates the pressure sensor from process fluids that are being sensed. Process fluids, which can be highly corrosive, are thus kept isolated from the pressure sensor in order to avoid corrosion or damage to the pressure sensor. Pressure is transferred from the isolation diaphragm to the pressure sensor using a substantially incompressible, inert fill fluid. The pressure sensor itself has a physical structure such as a sensing diaphragm that reacts to the pressure, such as by deforming. The pressure sensor also includes an electrical structure, such as a strain gauge or capacitive plate or electrode that reacts to the physical deformation. For example, some known pressure sensors have a deflectable diaphragm that bears a capacitive plate or electrode such that deflection of the diaphragm produces a change in the sensor's capacitance. However, a variety of other techniques are known.
Some process pressure transmitters operate in the vicinity of, or within, seawater. Accordingly, such marine pressure transmitters are subject to the significant corrosive effects of seawater. In order to provide a robust design that can operate for an acceptable product lifetime, certain design considerations become important. For example, selecting a material that is substantially impervious to the corrosive effects of seawater may provide a robust design, however, the material costs for exotic alloys that provide such protection may result in a cost prohibitive design. Titanium, for example, is completely impervious to seawater-induced corrosion, but has been found to be substantially impossible to weld with other alloys and materials, such as stainless steel. Moreover, it is difficult to solder the two materials together. Further still, a pressure transmitter built completely from titanium is not cost effective.
Providing a cost effective process pressure transmitter that is adapted for prolonged exposure to seawater would provide an important improvement for marine-based process control environments.